Heretofore, a high-power brushless motor has been employed in a hybrid electric vehicle or an electric vehicle. To control the brushless motor in the hybrid electric vehicle, it is necessary to accurately detect the rotational position of an output shaft of a motor. This is because the rotational position (a rotation angle) of a rotor has to be detected in order to control switching of energization to each coil. In vehicles, particularly, cogging is apt to deteriorate driveability and hence there is a demand for reducing such cogging. For this end, accurate switching of energization is requested.
For detecting the position of a motor shaft of a vehicle, a resolver is used because of good high heat resistance, noise resistance, vibration resistance, high humidity resistance, etc. The resolver is incorporated in the motor and directly attached to the rotor shaft.
For instance, a resolver of Patent Literature 1 is arranged to amplitude modulate a high-frequency wave with a sine wave and a cosine wave and then input the amplitude-modulated high-frequency wave as an excitation signal to an excitation coil. This can provide an effect of reducing the number of turns of each excitation coil. Herein, the sine wave, the cosine wave, and the high-frequency wave are analog waves.
Furthermore, Patent Literatures 2 to 4 disclose that an output signal detected and output by a detection coil is converted to digital.